Rhipicephalus microplus, better known as the Asiatic cattle tick, is a largely invasive
ectoparasite of great economic importance due to the negative effect it has on agricultural
livestock on a global scale, particularly cattle. Tick-borne diseases (babesiosis and anaplasmosis)
transmitted by R. microplus are alarming as they decrease the quality of livestock
health and production. In sub-Saharan Africa, cattle represent a major source of meat and
milk, but this region of the world is severely affected by the Rhipicephalus microplus tick.
The principal method for tick control is the use of chemical acaricides, notably amitraz,
which was implemented in the 1990’s after resistance to other acaricides surfaced. However,
the efficiency of chemical control is hindered by an increase in the frequency of mutant
resistance alleles to amitraz in tick populations. Presently, the only way to assess amitraz
resistance is by means of larval packet tests, but this technique is time-consuming and not
particularly cost effective. The main aims of this study were three-fold. First, we attempted
to correlate two known SNPs in the octopamine/tyramine (OCT/Tyr) receptor with amitraz
resistance in South African field samples of R. microplus. Second, we calculated gametic
disequilibrium for these SNPs to determine whether they are randomly associated. Lastly,
we conducted a study to assess the evolutionary effects of recombination within the OCT/
Tyr receptor. Our results confirmed that the two SNPs are associated with amitraz resistance
in the South African tick strain, and that they are in gametic disequilibrium. Additionally,
recombination was detected in the OCT/Tyr receptor generating two recombinant haplotypes. These results are of concern to farmers in sub-Saharan Africa, and the emergence
of amitraz resistance should be closely monitored in future. Therefore, we present a
quick and affordable RFLP based diagnostic technique to assess amitraz resistance in field
samples of R. microplus.
S1 Fig. Subpopulation structure of ticks across South Africa. Ticks from each farm were
placed into subpopulations (1–15) depending on the region from which they were collected.
Grid blocks were constructed 300 x 300 km over the country for accurate overall segregation of
populations. The farms from which tick samples were analyzed are indicated in the table, along
with their grid block number and province. Farm numbers correspond with sample number,
e.g. sample 44.1MF is sample 1 of female R. microplus from farm 44.
S1 Table. GenBank accession numbers for all R. microplus OCT/Tyr receptor sequences.
S2 Table. Genotypes of field samples of R. microplus ticks at the two published SNP positions.
S3 Table. Rhipicephalus microplus larval packet test results.